Referring to FIGS. 1a and 1b, computerized tomography (CT) involves the imaging of the internal structure of an object 102 by collecting several projection images (“radiographic projections”) in a single scan operation (“scan”), and is widely used in the medical field to view the internal structure of selected portions of the human body. Typically, several two-dimensional projections are made of the object at different projection angles, and a three-dimensional representation of the object is constructed from the projections using various tomographic reconstruction methods. From the three-dimensional image, conventional axial, coronal, or sagittal CT slices through the object can be generated. The two-dimensional projections are typically created by transmitting radiation from a “point source” 105 through the object 102, which will absorb some of the radiation based on its size, density, and atomic composition, and collecting the non-absorbed radiation onto a two-dimensional imaging device, or imager 110, which comprises an array of pixel detectors (simply called “pixels”). In the typical CT system, the radiation source 105 and imaging detector 110 are mounted on a gantry 150 which rotates them around the object 102 being scanned. Such a system is shown in FIG. 1a. A line that goes through the point radiation source 105, the center of rotation (known as the isocenter), and is perpendicular to the two-dimensional imager 110 is called the projection axis 112. The point where the projection axis hits the detector is known as the piercing point of the detector. Typically, the detector piercing point is located at or near the center of the imager in full-fan geometry (also known as centered-detector geometry). Described below is a variation called half-fan (offset-detector) geometry, wherein the detector array is offset, thus allowing for an increase in the field-of-view (FOV) with a 360 degree scan rotation.
The source's radiation emanates toward the imaging device 105 in a volume of space defined by a right-circular, elliptical, or rectangular cone having its vertex at the point source and its base at the imaging device. For this reason, the radiation is often called cone-beam (CB) radiation. Most commonly, the beam is collimated near the source into a rectangular cone or pyramid so that, when no object is present, the beam radiation only falls on the detector and not outside the detector boundaries. Generally, when no object is present within the cone, the distribution of radiation is substantially uniform on the imager. However, the distribution of the radiation may be slightly non-uniform. In any event, any non-uniformity in the distribution can be measured in a calibration step and accounted for. The projection axis may not be at the center of the imager or the center of the object. It may pass through them at arbitrary locations including very near the edge.
FIG. 1b shows further aspects of the CT system of FIG. 1a, including gantry 150 and controller 155. Controller 155 is coupled to radiation source 105, imaging device 110, and user interface 115. User interface 115 provides a human interface to controller 155 that enables the user to at least initiate a scan of the object, and to collect measured projection data from the imaging device. User interface 115 may be configured to present graphic representations of the measured data.
In an ideal imaging system, rays of radiation travel along respective straight-line transmission paths from the source, through the object (where they are partially absorbed), and then to respective pixel detectors without generating scattered rays that are detected. However, in real systems, when a quantum of radiation is absorbed by a portion of the object, one or more scattered rays are often generated that deviate from the transmission path of the incident radiation. These scattered rays are often received by “surrounding” detector elements that are not located on the transmission path that the initial quantum of radiation was transmitted on, thereby creating measurement errors.
The measurement errors created by scattered radiation cause artifacts and loss of spatial and contrast resolution in the radiographic projection data and the CT images produced by the radiation imaging system. The scattered radiation can also cause numerical errors in the image reconstruction algorithms (generally referred to as “CT number problems” in the art). All of the foregoing leads to image degradation.
Scattered radiation may arise from many sources. These may include: the bow-tie filter (if present), the object being scanned, an anti-scatter grid (if present), and the detector housing. One model for addressing these aforementioned scattering sources is described in U.S. patent application Ser. No. 12/125,053, filed on May 21, 2008, corresponding to patent publication number 20090290682, published Nov. 26, 2009. This application is hereby incorporated by reference in its entirety.
As indicated by both experiment and by Monte Carlo simulations, different types of objects can have different scattering properties and if they are all in the imaging field-of-view at the same time, then they must be appropriately addressed by a scatter correction method. For example, a significant amount of scatter can come from objects (e.g., supporting structures) adjacent to the actual object(s) of interest (e.g., the human body). One type of supporting structure is the patient table which is commonly made of polycarbon, a light material having a high probability of Compton interactions. It has been found that the scatter correction method described in the incorporated application may not be optimized to effectively model scatter from sources such as the patient table, due to how its shape, position, density, and material composition differ from that of the human body. As a result, scatter is underestimated for some projections, and a cupping artifact is observed in patients below the isocenter region when using the half-fan geometry with an offset detector (described and shown in FIG. 2). A method and system for estimating and correcting for scatter from multiple types of objects including the patient table is described hereinafter.